Hi! I am new here and not the most experienced person, but I'd love to give some feedback if I can

I think this is an interesting concept that could do great if it was built up some more! I was a little confused with the slightly ambiguous background and the description of where these creatures were found. I wasn't personally very interested in them, since they do not seem to show much unique qualities (I believe the main one was eating metal)

However I think if it was worked around, and some more interesting anomalous properties were added it could be good! :)

All captured instances of SCP-XXXX must be contained within their own individual glass test tubes. Each test tube must have a diameter of at least thirty (30) mm.

I would blend these two sentences into one: "All captured instances of SCP-XXXX are to be contained within individual glass test tubes with a minimum diameter of 30 mm."

The reason for destroying instances when class a or b personnel work in the facility is not apparent in the rest of the article. I would suggest either cutting it or giving a reason for its existence.

Most instances of SCP-XXXX are mechanical arthropods

There are never any instances that are described differently, so remove the word "most".

I would describe what the cores look like before talking about where the majority of their mass is.

Instances have been discovered in obscure places such as oceans and outer space, and have displayed signs of adaptation according to their environments, such as jet drive engines or self-propulsion, respectively.

Reword to read "Instances have also been discovered in oceans and outer space." I would cut this into two sentences at the "space, and" but it's up to you.

They are often perceived as pests by citizens if they do not report the unusual sighting, at which the reported instance is shortly captured and contained at a Foundation site.

I would reword this to read "They are often perceived as pests by citizens." I would then word the capture bit to this: "If a citizen reports a sighting of an instance, the instance is to be captured and contained." and put it in the containment procedures

They may be found nearby and sometimes within Foundation facilities, especially Site-19.

Reword this to read "They are also found nearby and within Foundation facilities, most commonly Site-19."

that correspond to the materials of itself

I would change this to read "that correspond to the materials the instance is made of", as it confused me on first read.

This cycle of exponential construction of copies is largely the reason for their pest-like existence.

I don't think you need this sentence, as it doesn't feel like something that would belong in an official document.

The first instance of SCP-XXXX was discovered near Site 19 on 11-4-2000, where it was then captured and assigned SCP-XXXX; the discovery was followed by three (3) more encounters, and they were assigned SCP-XXXX-2 through -4.

I think you could cut this down to just read "The first instance of SCP-XXXX was discovered near Site 19 on 11-4-2000." as the rest feels redundant.

You have a pretty good grasp of clinical tone, and I'm intrigued by the concept. However, a lot of the description of how they copy each other feels like it could be cut down, but I'm not sure how to do that, so it's probably best to leave it for now. The addendum would work better for me if it was worded clinically, as it's extremely interesting. The number (#) (Example: three (3))is not necessary. If the number is ten or lower, spell it out. If it's higher than ten, use the #.

This would work better for me if more was expanded upon with the fact that this scp seems to have been created to spy on the foundation, apparently by this "BKD". Expansion upon that idea would definitely help give this scp more of a narrative. It would also make it more interesting if these were just found near Foundation sites, prompting a more intense investigation into the origin of these things that could help you construct a narrative.

I have reworked some major areas in the draft according to your critiques, particularly the addendum and the method for how each instance creates a duplicate of itself. I put off explaining the "BKD" until I can get a firm grip on an interesting narrative; I'll add more about it once I do. I await any additional critique.

I have begun to put together a description of the entity that "BKD" stands for, and have placed it on the fourth tab of the draft page. I know I should probably be writing this in a separate forum, but I am confirming a connection with the two drafts so that there is little to no confusion regarding such possible connection. Any advice or critique is appreciated.